The present invention relates to process field devices of the type used to monitor or control an industrial process. Specifically, the present invention relates to process field devices which are powered with power received from a two-wire industrial control loop.
Industrial processes are used in a variety of applications. For example, such systems are used to produce or control process fluids. Examples include oil refineries, food production, paper pulp production, etc. In such industrial processes, process fluids are utilized. Various process variables of the process fluid are monitored or controlled. Examples process variables include temperature, pressure, flow rate, level, etc. In industrial processes, process field devices are used to measure and/or control the process variables. A field device which measures a process variable is often referred to a process variable transmitter. A field device which is used to control a process variable may be referred to a process variable controller.
In many industrial processes, the process field devices are in communication with a centralized location such as a control room. Process variables are transmitted to the control room and equipment in the control room can be used to control the process by transmitting control signals to process variable controllers. Many industrial process installations utilize two-wire industrial process control loops for communication with process field devices. In such a configuration, the current level on the process control loop can be set to a value which represents a measured process variable, or set to value which represents a desired value for controlling a process variable. One example is a two-wire process control loop which operates in accordance with the HART® communication protocol standard. In such a configuration, the current level through the process control loop can be controlled to represent a process variable and additional digital information is transmitted by modulating a digital signal onto the analog current level.
Many process field devices are configured to be powered from the same two-wire process control loop used to transmit information. The two-wire process control loop may couple to a single field device or to multiple field devices (“multi-drop”). This limits the power which is available to the process field device. In many instances, it is desirable to maximize the amount of power available to circuitry of the process field device.